Red cabbage pH indicator
Recommended age: 8+. Adult supervision required! We brew a homemade pH indicator from red cabbage that turns red in acidic solutions and green in basic ones — making chemistry visible right in your kitchen.
Chemicals
- 200 g red cabbage (about a quarter of a small head, chopped) — about a quarter of a small head, chopped
- 500 ml water (plain tap water) — plain tap water
- 50 ml household vinegar (10%) (the first test solution) — the first test solution
- 50 ml lemon juice (freshly squeezed or bottled) — freshly squeezed or bottled
- 1 tsp baking soda (stirred into half a glass of water) — stirred into half a glass of water
- 1 tsp liquid soap (stirred into half a glass of water) — stirred into half a glass of water
Procedure
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The hot liquid can scald: boiling and straining are the adult's job! Work with your goggles on, never taste the test solutions, and do not bring strong cleaning products into the game.
Safety! -
Chop the red cabbage into smaller pieces — if you need a knife, ask the adult for help.
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The adult boils the cabbage in 500 ml of water for about 10 minutes, until the liquid turns a deep purple.
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Let it cool, then the adult strains the liquid into the jar. This is your indicator!
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Line up five glasses and pour a finger's depth of indicator into each. The first one stays untouched as your reference.
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Add a spoonful of vinegar to the second glass and lemon juice to the third — watch the purple turn red!
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Spoon the baking-soda solution into the fourth glass and the soap solution into the fifth — these turn the indicator blue, then green.
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If you soak blotting paper or a coffee filter in the indicator and let it dry, you get your own pH paper, ready any time for a one-drop test.
Technique -
Arrange the glasses in colour order from red to green and note down which substance is acidic or basic — you have made your own colour scale.
Apparatus
- 1× saucepan — 1,5–2 l
- 1× sieve
- 6× small clear glass
- 2× teaspoon
- 1× jar for storage — kb. 0,5 l
Frequently asked questions
- Why does it work?
- The purple pigment of the cabbage, anthocyanin, is a molecule whose structure — and with it, its colour — changes when it meets an acid or a base. It turns red in acidic solutions, stays purple in neutral ones and goes blue-green in basic ones, so it signals pH precisely.
- What other plants does it work with?
- With any plant rich in anthocyanins: blueberry or blackcurrant juice, a brew of red onion skins, even purple sweet potato. It is fun to compare which gives the prettiest colour transitions.
- How long does the indicator keep?
- Three to four days in a closed jar in the fridge. If you want to keep it longer, freeze it in an ice-cube tray — thawed cube by cube, it stays usable for months.
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